1. Field of the Invention
The present invention relates to a flash device having a flash discharge tube which flashes based on an electric energy charged in a main capacitor, and more particularly to a flash device that allows to choose whether the flash device is to fire or not when a triggering voltage is applied to the flash discharge tube upon a triggering switch being turned on synchronously with a shutter release of a camera.
2. Background Arts
When the subject brightness is so low that a proper exposure would not be provided without any artificial illumination, a flash device is often used to project light toward a photographic subject synchronously with releasing the shutter. Since compact cameras and lens-fitted photo film units have an inexpensive lens system of a relatively large f-number, it is desirable to provide them with a built-in flash device. To enable the flash device to fire, it is necessary to charge a main capacitor of the flash device up to a set voltage prior to the shutter release. Conventionally, a flash charge switch is previously turned on to start charging the flash device when to make a flash photography.
A flash device is also known in the art where the photographer can choose whether to fire the flash device or not by setting a switching member to an ON position or an OFF position respectively, for example, by Japanese Utility Model Registration No. 2,535,884 and U.S. patent application No. 08/933,984, now U.S. Pat. No. 5,966,552. Hereinafter, this type of flash device will be called an ON-OFF type flash device.
FIG. 4 shows a flash circuit of the ON-OFF type flash device disclosed in the above mentioned Japanese Utility Model Registration. In this flash circuit, an oscillation transistor 29 and an oscillation transformer 30 constitute a well-known blocking oscillator that transforms a low-level power source voltage from a battery 19 into a high-level voltage used for charging a main capacitor 16. When a biasing current from the battery 19 flows through a tertiary coil 33 of the oscillation transformer 30 into a base of the oscillation transistor 29, the oscillation transistor 29 lets a collector current or a primary current flow through a primary coil 31 of the oscillation transformer 30. Because the primary current induces an electromotive force on a secondary coil 32 of the oscillation transformer 30, a secondary current flows through the secondary coil 32. As the secondary current is fed back to the base of the oscillation transistor 29, the oscillation transistor 29 oscillates and amplifies the primary current. As a result, the primary current increases up to about 5A or so. During this oscillation, the main capacitor 16 is charged with the secondary current at a high voltage level.
A flash charge switch 67 is connected to an emitter of the oscillation transistor 29. The flash charge switch 67 is turned on or off to conduct or not conduct the biasing current from the battery 19, and thus the primary current and the secondary current. Thereby, the flash charge switch 67 is used to choose whether the oscillating transistor 29 should oscillate for charging or not. A flash ON-OFF switch 68 is connected between the emitter of the oscillating transformer 30 and a triggering capacitor 35. The flash ON-OFF switch 68 is turned on and off by the same switching member that is operated to turn the flash charge switch 67 on and off. At one switching position of the switching member, both of the switches 67 and 68 are on, whereas these switches 67 and 68 are off at the other switching position.
While the flash ON-OFF switch 68 is on, the secondary current flows through the flash ON-OFF switch 68 and charges the triggering capacitor 35, and the triggering capacitor 35 may discharge the current upon a synchronized triggering switch 18 being turned on. As the current discharged from the triggering capacitor 35 flows into a primary coil of a triggering transformer 37, a triggering voltage is applied to a flash discharge tube 38. Then, the main capacitor 16 is discharged through the flash discharge tube 38. Thus, the flash device fires. While the flash ON-OFF switch 68 is off, even when the triggering switch 18 is turned on, since the current does not flows from the triggering capacitor 35 into the triggering transformer 37, the flash device does not fire.
According to the circuit of FIG. 4, each of the flash charge switch 67 and the flash ON-OFF switch 68 is connected directly to the emitter of the oscillation transistor 29, so that these switches 67 and 68 may be constituted of a three-terminal contact having a fixed common terminal and two respective terminals. The three-terminal contact contributes to reducing the space for mounting these switching 67 and 68, and the number of elements and thus the number of manufacturing processes of the flash device.
FIG. 5 shows a flash circuit of the ON-OFF flash device disclosed in the above mentioned U.S. Application, wherein a flash charge switch 67 is connected between a plus pole of a battery 19 and a tertiary coil 33, whereas a flash ON-OFF switch is connected in series to a triggering capacitor 35. Also in this flash circuit, since one terminal of the flash charge switch 67 is connected directly to one terminal of the flash ON-OFF switch 68, the flash charge switch 67 and the flash ON-OFF switch 68 may be constituted of a three-terminal contact.
The circuit of FIG. 5 further has an oscillation stopping circuit 40 for deactivating the blocking oscillator when the main capacitor 16 is charged up to the set voltage, and a light emitting diode 48 that starts lighting to indicate completion of charging when the main capacitor 16 is almost charged up to the set voltage.
FIG. 6 shows a modification of the flash circuit of FIG. 5, wherein a flash charge switch 67 is connected between a plus pole of a battery 19 and a connecting point of primary and tertiary coils 31 and 33. Other constructions are the same as shown in FIG. 5. Also in this circuit, the flash charge switch 67 and a flash ON-OFF switch 68 may be constituted of a three-terminal contact.
According to the flash circuit of FIG. 4, the flash charge switch 67 is connected in a primary circuit of the blocking oscillator, through which the primary current flows. The primary circuit is from a plus pole of the battery 19 through the primary coil 31 and the emitter-collector circuit of the oscillating transistor 29 to a minus pole of the battery 19. As the primary current flows through the flash charge switch 67, the primary current is reduced by a contact resistance of the flash charge switch 67. The reduction of the primary current results in a reduction of the secondary current, so that electric energy supplied per a given time to the main capacitor 16 is reduced, so it takes longer to charge the main capacitor 16 up to the set voltage than without the flash charge switch 67. The same problem occurs wherever the flash charge switch 67 is connected in the primary circuit, for example, as shown in FIG. 6.
Because the flash charge switch 67 is not connected in the primary circuit of the blocking oscillator in the flash circuit of FIG. 5, there is not any problem of the contact resistance. However, even while the flash charge switch 67 is off, the flash charge switch 67 just stops the bias current from being supplied to the oscillating transistor 29, whereas the primary coil 31 is kept being connected to the battery 19, and the secondary current can flows from the secondary coil 32 into the base of the oscillating transistor 29, so that the oscillating transistor 29 oscillates by itself. The oscillating transistor 29 does not stop oscillating and thus charging until the oscillation stopping circuit 40 is activated as the main capacitor 16 is fully charged. Since the light emitting diode 48 continues to light so long as the main capacitor 16 is almost fully charged, even after the flash ON-OFF switch and the flash charge switch are turned off concurrently, the photographer may misunderstand that the flash device is ready to flash, or something goes wrong with the flash device.
The above problems with the flash circuits of FIGS. 4 to 6 are solved by connecting a flash charge switch 67 to a base of an 29 as shown in FIG. 7. In the flash circuit of FIG. 7, however, since a flash ON-OFF switch 68 is connected between an emitter of the oscillating transistor 29 and a triggering capacitor 35, it is impossible to constitute these switches 67 and 68 of a three-terminal contact, and it is necessary to provide a two-terminal contact for each of these switches 67 and 68 separately.
Therefore, the flash circuit of FIG. 7 needs a larger mounting space for these switches 67 and 68, as well as a larger number of manufacturing processes. Thus the manufacturing cost inevitably increases. When operating these switches 67 and 68 concurrently, the increase in the number of terminals and contact members results in increasing the probability of contact failure and other malfunction of these switches 67 and 68.
In view of the foregoing, an object of the present invention is to provide a flash device that uses a three-terminal contact commonly for a flash charge switch and a flash ON-OFF switch, but does not take a long time to charge up, and can stop charging quickly.